A. Field of the Invention
This invention relates to processes for preparing polymers by the free-radical polymerization of vinyl monomer(s) using a Redox catalyst system comprising oxidizing agent, reducing agent and metal ions (e.g. iron ions) as promoter. More particularly, the invention relates to stopping and restarting such processes without forming off-specification polymer. By "off-specification" polymer is meant polymer having properties, such as specific viscosity, significantly different from the polymer desired. By "significantly different" is meant a difference which adversely affects the utility of the polymer, for example, its dyeability or strength in fiber form.
B. Description of the Prior Art
Processes for the free-radical polymerization of vinyl monomer(s) using the above-mentioned Redox catalyst system are well-known in the art, for example, the aqueous dispersion polymerization of acrylonitrile with one or more vinyl monomers to form acrylic polymer. Commercially, this process is carried out by continuously introducing feed materials (monomers, Redox catalyst components, water, etc.) to a stirred reactor and continuously withdrawing an aqueous stream comprising dispersed acrylic polymer, unreacted monomer and catalyst components from the reactor, usually, by permitting the contents of the reactor to flow into a conduit located near the top of the reactor. An agent is added to the withdrawn stream to shortstop the polymerization reaction and permit recovery of the unreacted monomer. A Redox catalyst system which may be used with this process comprises persulfate ions (.sup.= S.sub.2 O.sub.8) as oxidizing agent, bisulfite ions (.sup.- HSO.sub.3) as reducing agent and iron ions (Fe.sup.++ and Fe.sup.+++) as promoter. Reactions involved with use of such a system include: EQU .sup.= S.sub.2 O.sub.8 +Fe.sup.++ .fwdarw..sup.= SO.sub.4 +.sup.- SO.sub.4.+Fe.sup.+++ ( 1) EQU .sup.- HSO.sub.3 +Fe.sup.+++ .fwdarw.HSO.sub.3.+Fe.sup.++ ( 2) EQU HSO.sub.3.+M.fwdarw.HSO.sub.3 M. (3) EQU HSO.sub.3 M.+M.fwdarw.HSO.sub.3 M.sub.2., etc. (4) EQU .sup.- SO.sub.4.+M.fwdarw..sup.- SO.sub.4 M. (5) EQU .sup.- SO.sub.4 M.+M.fwdarw..sup.- SO.sub.4 M.sub.2., etc., (6)
where M represents vinyl monomer and M. represents vinyl monomer radical. The iron ions simultaneously react with oxidizing agent (1) and reducing agent (2) to form radicals that initiate polymerization of the vinyl monomer (3) and (4) and also (5) and (6). The normal commercial practice for halting or stopping this process is simply to stop all feed material to the reactor when required, for example, when a mechanical failure occurs or when preventive maintenance is needed. The process is normally restarted by simply once again introducing the feed materials to the reactor. This method of stopping and restarting the process causes large amount of off-specification polymer to be formed when the polymerization is stopped and again when it is restarted. The off-specification polymer has unacceptable dyeing characteristics for fiber applications. Segregation of off-specification polymer from polymer within specifications is a costly and time consuming operation. Typically, upon restarting the process, the aqueous stream withdrawn from the reactor is discarded until all the polymer is once again within specifications.